Grinding machine calibration

ABSTRACT

A device and method for the setup, alignment and calibration of grinding machines is disclosed. A tool ( 42   c ) is described, comprising a bar ( 44 ) and a first location feature ( 48 ) located on the bar ( 44 ) and configured to engage with a first location of a machine. The tool also provides a datum ( 66 ) located at a known but variable distance parallel to the bar ( 44 ) from the first location feature ( 48 ). A method for calibrating a grinding machine using the tool ( 42   c ) is also described.

The present disclosure concerns the set up and calibration of grindingmachines. More specifically, it relates to a device and method foraligning and calibrating a Blade Tip Grinding Machine (BTG) for use oncompressor rotor blades of a Gas Turbine engine.

In order to obtain optimum performance from a Gas Turbine engine, it isnecessary to control the tip clearance between the tips of bothintermediate pressure (IP) and high pressure (HP) compressor rotorblades and their corresponding rotor paths in their respective casings.Optimum tip clearance can be achieved by grinding the blades of the IPand HP rotors on a purpose-built Blade Tip Grinding Machine (BTG)following their assembly into the rotor drum.

BTG machines are known, and are manufactured by several differentcompanies. Typically, they incorporate some form of measurement systemto determine the radii of rotor blade tips during and post grinding.Some of the typical measurement systems used on BTG's are notparticularly accurate, temperature stable or reliable, and hence someBTG machines now incorporate an optical measurement system to improvethe accuracy and reliability of measurements made during use of themachine.

Blade Tip Grinding Machines (BTG) grind rotors to a finished size in amanually or automatically controlled closed loop grinding cycle, wherethe tip radii determined by the optical gauge “in process” controls thein-feed of the grinding wheel on the machine as part of the controlsystem.

However, in order for rotors to be ground to the correct size it isessential that the measurement plane and direction of travel of theoptical gauge and the slide system on which it is mounted are alignedand positioned correctly to the axis of rotation and the datum featuresof the grinding machine. This can be difficult to achieve withconventional measurement and alignment equipment.

It is an aim of the present invention to address, mitigate, or overcomesome or all of these difficulties.

According to a first aspect there is provided a machine calibrationtool, the tool comprising a bar which has a longitudinal axis; a firstlocation feature which is located on the bar and configured to engagewith a first location of the machine; and, a datum having a locationadjacent to the bar at a known but variable distance parallel to thelongitudinal axis from the first location feature.

The tool is intended to be used with an optical sensor such as a ZIMMERgauge or alternative measuring system or device to determine thelocation of machine mounted datums, for example knife-edge datums, on agrinding machine relative to machine datums, such as a rotating axis ofthe machine and/or a spindle face.

The perpendicular distance of the datum from the longitudinal axis ofthe bar may be known at all distances of the datum parallel to thelongitudinal axis from the first location feature.

The datum may comprise one or more reference features.

One or more of the reference features may be tapered along thelongitudinal axis of the bar and/or parallel to the longitudinal axis ofthe bar, and/or perpendicular to the longitudinal axis of the bar.

One or more of the reference features may comprise one or moreknife-edges, for example provided on a knife-edge square plate.

A thin knife-edge can be used to represent a typical blade tip which isto be ground. A tapered interface, for example a knife-edge squareplate, allows a very precise and accurate measurement.

A plate with two perpendicular sides can be provided to form a squareedge. The two perpendicular sides allow the accurate/precise measurementof both axial and radial positions of the cutting edge of a grindingwheel of a grinding machine with respect to the reference point, forexample corresponding to the axis of rotation.

The edge can be used to get an accurate position of the edge by acamera/Laser projecting a beam of light into/onto the edge. Theresulting shadow of the tapered interface can be used as themeasurement. The two faces of the plate may be tapered over a distanceof around 2 mm.

The bar may comprise a reference face which is parallel to thelongitudinal axis of the bar, to which the datum is engaged.

The bar may be comprised of one or more of a metal, alloy, ceramic,stone (such as granite) or polymer.

The datum may be fixedly attached to the bar at any one of two or morelocations parallel to the longitudinal axis of the bar.

Alternatively, the datum may be slidably attached to the bar and beselectively movable parallel to the longitudinal axis of the bar. Thedatum may be fixable in any desired position parallel to thelongitudinal axis of the bar, for example by a clamping arrangementcomprising a T-slot provided in a face of the bar.

The datum may be configured within an arm arrangement.

The datum may be configured to pivot about a pivotable attachmentfeature within the arm arrangement.

The arm arrangement may be comprised of one or more of a metal, alloy,ceramic, stone (such as granite) or polymer.

The machine calibration tool may further comprise a clocking mandrellocated on the bar and aligned with the first location feature.Providing a clocking mandrel, or a tube or other extension from a frontface of the bar, provides an accessible reference point to measure theradial distance to the datum.

The clocking mandrel may be movable relative to the bar. In particular,the clocking mandrel may be adjustable for alignment with the workingaxis of a grinding machine.

A method of calibrating a grinding machine is also provided, comprisingthe steps of securing the first location feature of a machinecalibration tool according to any preceding claim to a grinding machine,setting a datum of the machine calibration tool at a known positionrelative to a fixed machine datum feature, and using a measurementdevice and the datum of the machine calibration tool to determine theposition of one or more further datum features mounted to the grindingmachine relative to the machine datum feature.

The measurement device comprises an optical sensor, possibly associatedwith or mounted on the grinding machine. Alternatively, the measurementdevice may comprise a triangulation probe or a physical measurementdevice.

The one or more further datum features mounted to the grinding machinemay comprise machine mounted datum knife-edges.

The grinding machine may be a blade tip grinding (BTG) machine, and themachine datum feature may comprise a rotational axis of the BTG machine,for example the rotational axis of a machine workhead spindle and/orspindle face. Other possible machine datum features such as a tailstockspindle or spindle face may alternatively, or additionally, be includedin the calibration.

It is envisaged that the invention will be used during the installationof new gauges, following refurbishment, and for the routine calibrationof optical sensors such as ZIMMER systems or similar/derivatives on BTGmachines.

The skilled person will appreciate that except where mutually exclusive,a feature described in relation to any one of the above aspects may beapplied mutatis mutandis to any other aspect. Furthermore, except wheremutually exclusive any feature described herein may be applied to anyaspect and/or combined with any other feature described herein.

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a schematic view of a compressor drum in a BTG machine;

FIG. 3 is a schematic view of a calibration tool held in the chuck of aBTG machine;

FIG. 4 is a schematic view of the calibration tool of FIG. 3 inisolation;

FIG. 5 is a schematic view of an alternative calibration tool;

FIG. 6 is a schematic view of a further alternative calibration tool;

FIG. 7 is a schematic view of a further alternative calibration tool;and

FIG. 8 is a schematic view of a further alternative calibration tool.

With reference to FIG. 1, a gas turbine engine is generally indicated at10, having a principal and rotational axis 11. The engine 10 comprises,in axial flow series, an air intake 12, a propulsive fan 13, anintermediate pressure compressor 14, a high pressure compressor 15,combustion equipment 16, a high pressure turbine 17, an intermediatepressure turbine 18, a low pressure turbine 19 and an exhaust nozzle 20.A nacelle 21 generally surrounds the engine 10 and defines both theintake 12 and the exhaust nozzle 20.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 12 is accelerated by the fan 13 to produce two airflows: a first air flow into the intermediate pressure compressor 14 anda second air flow which passes through a bypass duct 22 to providepropulsive thrust. The intermediate pressure compressor 14 compressesthe air flow directed into it before delivering that air to the highpressure compressor 15 where further compression takes place.

The compressed air exhausted from the high pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate and lowpressure turbines 17, 18, 19 before being exhausted through the nozzle20 to provide additional propulsive thrust. The high 17, intermediate 18and low 19 pressure turbines drive respectively the high pressurecompressor 15, intermediate pressure compressor 14 and fan 13, each bysuitable interconnecting shaft.

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. By way of example such engines mayhave an alternative number of interconnecting shafts (e.g. two) and/oran alternative number of compressors and/or turbines. Further the enginemay comprise a gearbox provided in the drive train from a turbine to acompressor and/or fan.

FIG. 2 shows an intermediate pressure (IP) compressor drum, generallyindicated 14, in a blade tip grinding (BTG) machine, which is generallydesignated 24. The IP compressor drum 14 is mounted on a central axis 26between a headstock spindle 28 and a tailstock spindle 30 of the BTGmachine 24. A grinding wheel 32 and an optical sensor 34 are alignedwith one stage 36 of the compressor drum 14. As the compressor drum 14is rotated about the axis 26, the grinding wheel 32 is moved intocontact with the ends of individual compressor blades in a particularstage 36, while the optical sensor 34 measures the location of the bladetips, and thereby determines their radii from the axis 26.

It will be understood that in order for the BTG machine 24 to ensurethat the rotor/blades are machined to the required finished size, andthus ensure the correct tip clearance when the drum 14 is received inits compressor casing, the optical sensor 34 must be aligned andpositioned correctly relative to the axis of rotation 26 and other datumfeatures of the BTG machine 24.

Machine mounted calibration edges such as datum knife-edges 38, 40 arealso provided at known radial and axial distances from the datumfeatures forming part of the BTG machine 24 structure, such as therotational axis 26 of the workhead spindle 28 and/or the tailstockspindle 30, and are used during machining of blades on the rotor/drum14.

FIG. 3 shows a calibration tool 42 mounted in the BTG machine 24 of FIG.2. The calibration tool 42 will be described more fully below, butbriefly comprises a gauge bar 44, a gauge arm 46 extending at rightangles to the gauge bar 44, and a mounting spigot 48 via which thecalibration tool 42 can be mounted to the headstock spindle 28. Theoptical sensor 34 is aligned with a free end portion of the gauge arm46, which represents the position of the tip of a machined compressorblade.

The optical sensor 34 is capable of measuring in the plane perpendicularto its optical axis, which is into the page as shown, so can detectdatum features provided on the gauge arm 46. Through use of thecalibration tool 42, the position of the machine mounted datumknife-edges 38, 40 may be reliably established relative to the datumfeatures forming part of the BTG machine structure.

An optical sensor, such as a ZIMMER gauge, may be aligned axially andradially relative to datum features on the BTG machine using thedescribed calibration tool. Some blade stages have “Hade” angles groundon their tips, and therefore any axial positioning error can affect themeasured radii of individual blade stages.

The calibration tool 42 from FIG. 3 is shown in greater detail in FIG.4.

The gauge bar 44 is flat and straight, with the sides square to eachother, and is of sufficient rigidity and stability such that it does notdistort under its own weight and that of the gauge arm 46. The gauge bar44 is provided with a number of discrete mounting points, stops or otherfixings to allow location of the gauge arm 46 at a number ofdiscrete/defined radial distances 60 from the mounting spigot 48. Thegauge bar 44 and arm 46 as illustrated are steel components, but itshould be understood that either or both could instead be constructedfrom an alternative thermally and dimensionally stable material, such asa suitable metal, alloy, ceramic, stone (such as granite) or polymer, orfrom a combination of such materials.

The gauge bar 44 is fastened onto the mounting spigot 48. The functionof the mounting spigot 48 is to locate the gauge bar 44 and arm 46 ontothe spindle face of the BTG machine 24 in place of the hydraulic chuckused during machining operations, and assist with stability andalignment. However, it would be possible instead to provide the gaugebar 44 with suitable features to allow it to be mounted into a chuck,collet or similar work holding feature, or for the mounting spigot 48 tobe integrally formed with the gauge bar 44.

A rear engagement face 50 of the mounting spigot 48 abuts to the frontface of the spindle of the BTG machine 24, and a calibrated dimension 52between the plane of the rear face 50 of the mounting spigot 48 and thefront face 54 of the gauge bar 44 can then be established via a suitablemeasurement method.

The function of the gauge arm 46 is to provide datum features in knowngeometric alignment to the face 54 of gauge bar 44 and the rotationalaxis 26 of the BTG machine workhead spindle 28. For example, taking acorner 56 of the gauge arm 46 as a datum, the axial distance 58 from theface 54 is known, and the radial distance from the rotational axis 26 isthe sum of the distance 60 and the radius of the mounting spigot 48,both of which are known.

Levelling screws may be provided to set the gauge bar 44 parallel to thedirection of travel of the optical sensor 34 and the base of the BTGmachine. This operation may be performed using an engineer's spirit orelectronic level, mounted onto the horizontal surface of the gauge bar44. The centring of the mounting spigot 48 on the rotational axis 26 canbe checked using a Dial Test Indicator (DTI) or similar to measure therunout on its outside diameter (OD).

In order to account for possible misalignment of the tool 42 whensecured to the BTG machine, a clocking mandrel 51 is providedconcentrically with the mounting spigot 48 on the opposite side of thegauge arm 44. The alignment of the clocking mandrel 51 can be adjustedindependently of the gauge arm 44, and can thus be made preciselycoaxial with the rotational axis 26 of the BTG machine. This could beachieved by measuring the runout on the outside diameter of the clockingmandrel 51, for example with a Dial Test Indicator (DTI), and adjustingthe alignment of the clocking mandrel 51 so that the runout isminimised. A Total Indicated Runout (TIR) of less than 5 μm, forexample, would indicate that the outside diameter of the clockingmandrel 51 is concentric to the rotational axis 26 of the machine.

The clocking mandrel 51 gives greater certainty of the position of thedatum point 56 relative to the rotational axis 26. The distance 61 fromthe outer diameter of the clocking mandrel 51 to the datum point 56 canbe readily determined, either by direct measurement or by measurement toa known reference point on the gauge arm 46. The radius of the clockingmandrel 51 is known and, because of the possibility of fine adjustment,is known to precisely correspond to additional distance to therotational axis 26 of the machine.

The inclusion of the clocking mandrel 51 additionally provides a readilyaccessible reference for the machine axis 26, which is an importantdatum of the BTG machine that is largely obscured or inaccessible oncethe tool 42 is attached.

FIG. 5 shows an alternative calibration tool 42 a. A gauge bar 44,mounting spigot 48 and clocking mandrel 51 are provided as before, butin the alternative tool 42 a the gauge arm 46 has been replaced by ashaped extension 46 a having a surface 47 provided at an oblique angleto the front face 54 of the gauge bar 44. It will be understood that solong as the position and geometry of the shaped extension 46 a and itsposition relative to the gauge bar 44 are known, the axial position 58and radial position 60, 61 of any datum point 56 a on the angled surface47 can be readily determined. In other words, the location of the datum56 a perpendicular to the longitudinal axis of the gauge bar 44 is knownat all distances of the datum 56 a from the rotational axis 26 of theBTG machine 24.

A further alternative calibration tool 42 b is shown in FIG. 6. The tool42 b comprises a gauge bar 44, gauge arm 46, mounting spigot 48 andclocking mandrel 51 similar to the tool 42 of FIG. 4. However, thealternative calibration tool 42 b of FIG. 6 additionally comprises aknife-edge square plate 62, which provides a radial knife-edge 64 and anaxial knife-edge 66.

The radial knife-edge 64 and an axial knife-edge 66 provide featureswhich, when measured by the optical sensor 34, are of the same form asthe datum features 38, 40 of the BTG machine 24. Both the radialknife-edge 64 and the axial knife-edge 66 can be aligned in a plane thatpasses through the rotational axis 26, and is parallel to the top faceof the gauge bar 44. The optical sensor 34 is designed to “see”knife-edges, such as the datum knife-edges 38, 40 or compressor bladetips, so the inclusion of the knife-edge square plate 62 helps to ensurethat the datum(s) 56 b provided on the calibration tool 42 b is/arereliably detected. The radial and axial knife-edges 64, 66 provide agood approximation to compressor blade tips.

As noted above, the optical sensor 34 is capable of measuring in theplane perpendicular to its optical axis, i.e. along axes perpendicularto the radial and axial knife-edges 64, 66, and also of the machinemounted calibration edges 38, 40.

A further alternative calibration tool 42 c is shown in FIG. 7. The tool42 c comprises a gauge bar 44, mounting spigot 48 and clocking mandrel51 as before. The gauge arm 46 c in the tool 42 c of FIG. 7 extends atan oblique angle to the gauge bar 44 and is provided, at its free end,with a knife-edge square plate 62 as described in relation to FIG. 6.The axial knife-edge 66 of the knife-edge square plate 62 is aligned tobe parallel with the gauge bar 44.

In this alternative, the radial distance 60 c, 61 c from the mountingspigot 48 to a datum point is made continuously variable. The front face54 of gauge bar 44 is provided with a T-slot, and the gauge arm 46 c hastwo ground surfaces which abut to the gauge bar 44. Screws from thegauge arm 46 c are received in T-nuts fitted into the T-slot, allowingthe gauge bar 46 c to slide along the gauge bar 44 as shown at 68. Oncein the desired radial position, the ground surfaces of the gauge arm 46c are clamped against the ground surfaces of the gauge bar 44 bytightening the screws into the T-nuts. The sliding adjustment describedallows stepless, continuous, adjustment of the gauge arm 46 c in theradial direction. The radial distance 60 c may be set and measured usingany suitable measurement system and method.

FIG. 8 shows a further alternative calibration tool 42 d. The gauge arm46 d in this alternative has a first portion 70 extending from the gaugearm 44, and a second portion 72 mounted to the first portion 70 by apivot 74, and movable between positions indicated in broken lines. Aknife-edge square plate 62, as previously described, is provided on theend of the second portion 72 remote from the pivot 74. The axial andradial positions 58 d, 60 d, 61 d of the datum can both be varied usingthe pivot 74.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

For example, the gauge arm 46, 46 a, 46 b, 46 c, 46 d may be providedwith one of more measurement faces and/or levelling pads for mounting anengineer's spirit or electronic level to ensure that the gauge arm 46,46 a, 46 b, 46 c, 46 d is level relative to the base of the BTG machine24.

1. A machine calibration tool, the tool comprising: a bar which has alongitudinal axis; a first location feature which is located on the barand configured to engage with a first location of the machine; and, adatum having a location adjacent to the bar at a known but variabledistance parallel to the longitudinal axis from the first locationfeature.
 2. A machine calibration tool according to claim 1, wherein theperpendicular distance of the datum from the longitudinal axis of thebar is known at all distances of the datum parallel to the longitudinalaxis from the first location feature.
 3. A machine calibration toolaccording to claim 1, wherein the datum comprises one or more referencefeatures.
 4. A machine calibration tool according to claim 3, whereinone or more of the reference features are tapered along the longitudinalaxis of the bar.
 5. A machine calibration tool according to claim 3,wherein one or more of the reference features are parallel to thelongitudinal axis of the bar.
 6. A machine calibration tool according toclaim 3, wherein one or more of the reference features are perpendicularto the longitudinal axis of the bar.
 7. A machine calibration toolaccording to claim 3, wherein one or more of the reference featurescomprise one or more knife-edges.
 8. A machine calibration toolaccording to claim 1, wherein the bar comprises a reference face whichis parallel to the longitudinal axis of the bar, to which the datum isengaged.
 9. A machine calibration tool according to claim 1, wherein thebar is comprised of one or more of a metal, alloy, ceramic, stone orpolymer.
 10. A machine calibration tool according to claim 1, whereinthe datum is fixedly attached to the bar at any one of two or morelocations parallel to the longitudinal axis of the bar.
 11. A machinecalibration tool according to claim 1, wherein the datum is slidablyattached to the bar and is selectively movable parallel to thelongitudinal axis of the bar.
 12. A machine calibration tool accordingto claim 1, wherein the datum is configured within an arm arrangement.13. A machine calibration tool according to claim 12, wherein the datumis configured to pivot about a pivotable attachment feature within thearm arrangement.
 14. A machine calibration tool according to claim 12,wherein the arm arrangement is comprised of one or more of a metal,alloy, ceramic, stone or polymer.
 15. A machine calibration toolaccording to claim 1, further comprising a clocking mandrel located onthe bar and aligned with the first location feature. 25
 16. A machinecalibration tool according to claim 15, wherein the clocking mandrel ismovable relative to the bar.
 17. A method of calibrating a grindingmachine comprising the steps of: securing the first location feature ofa machine calibration tool according to claim 1 to a grinding machine,setting the datum of the machine calibration tool at a known positionrelative to a fixed machine datum feature, and using a measurementdevice and the datum of the machine calibration tool to determine theposition of one or more further datum features mounted to the grindingmachine relative to the machine datum feature.
 18. A method according toclaim 17, wherein the measurement device comprises an optical sensorassociated with the grinding machine.
 19. A method according to claim17, wherein the one or more further datum features comprise machinemounted datum knife-edges.
 20. A method according to claim 17, whereinthe grinding machine is a blade tip grinding (BTG) machine, and whereinthe machine datum feature comprises a rotational axis of the BTGmachine.